The invention relates to a display device comprising an electro-optical material between a first substrate provided with picture electrodes having reflecting parts, and a second transparent substrate provided with transparent picture electrodes with pixels at the area of overlapping parts of picture electrodes, and further comprising a color filter. Such display devices are used in, for example, (portable) display screens in hand-held telephones, organizers but also in, for example, automotive uses.
A (transflective) display device of the type described in the opening paragraph is described in WO 99/28782. In the reflective state, ambient light is reflected by the reflecting electrode of, for example, silver or aluminum. These electrodes pass light in the transmissive state. To this end, the reflecting material has apertures at the location of pixel elements. The color filter is provided across the electrodes or is provided across an electrode on the second substrate.
A first problem in these types of display devices is the fact that the color saturation in the transmissive part of a pixel usually differs from that in the reflective part, notably when the display device is based on a polarization effect. This is due to the fact that light from a backlight determines the image in the transmissive part, while the ambient light determines the image in the reflective part. The light from a backlight passes the color filter only once, whereas the ambient light passes the color filter twice due to reflection. Consequently, and due to the difference in light source (a backlight with light from, for example, a TL tube or LED or from more external light sources) different requirements as regards the transmissive part and the reflective part of the display device are imposed on the color filter.
Another problem occurs when birefringent material, for example, twisted nematic (liquid crystal) material is used in such a display device. The transmission/voltage characteristic is then not the same in transmission as in reflection.
It is, inter alia, an object of the present invention to eliminate one or more of the above-mentioned drawbacks.
To this end, a display device according to the invention is characterized in that, viewed transversely to the first substrate, and within a pixel, a color filter partly covers the reflecting part of a picture electrode. When used in reflection, light from the uncovered part of the picture electrode is mixed, as it were, with light from the part of the picture electrode covered by the color filter. The color saturation behavior can now be adjusted for reflection by varying the ratio between covered and uncovered surface so that it can be rendered substantially equal to that in transmission.
An additional advantage is that due to the difference in thickness of the layer of liquid crystal material, the transmission/voltage characteristic at the area of the part of the picture electrode which is not covered by the color filter has a different variation than at the area of the uncovered part. Consequently, these characteristics will be more equal to each other for transmission and reflection in, for example, a reflective LCD with a twist angle of 60xc2x0.
These and other aspects of the invention are apparent from and will be elucidated with reference to the embodiments described hereinafter.